1. Field of the Invention
This invention relates generally to a power amplifier employing a technique for spurious signal cancellation and, more particularly, to a power amplifier employing a feed forward error correction amplifier having a low IP3 value for generating greater magnitude intermodulation products to cancel spurious signals in an RF signal.
2. Discussion of the Related Art
Communications systems that transmit RF signals carrying information typically employ linear power amplifiers to amplify the transmit signal so that it has enough power to be received and deciphered by a receiver at a distant location. Nonlinearities within the power amplifier introduce signal harmonics, intermodulation products and other distortions and spurious counterparts of the RF signal that cause interfering signals to other frequency channels. The signal harmonics can typically be filtered out, but the intermodulation products and other spurious signals are generally too close to the main center frequency of the signal to be filtered. Thus, it is desirable to provide techniques to cancel the spurious signals produced by the power amplifier in the transmitter.
FIG. 1 is a schematic diagram of a known amplifier circuit 10 that amplifies an RF signal to be transmitted and cancels spurious signals in the amplified RF signal produced by the amplification process. The circuit 10 includes a signal splitter 12 that splits off a sample of the RF signal to be amplified and directs it onto a feed forward signal path 16. Most of the RF signal is directed onto a main signal path 14 where it is amplified for transmission by a power amplifier 18. In one example, the amplifier 18 is a high gain, high intermodulation performance amplifier that generates third order intermodulation products (IP3). The output power spectrum of the RF signal from the amplifier 18 includes both the amplified RF input signal and the third order intermodulation products produced by the amplification process. The amplified RF signal from the amplifier 18 is applied to a directional coupler 20 that couples off a portion of the amplified RF signal that includes the intermodulation products. The magnitude of the main frequency of the RF signal coupled off by the coupler 20 is about the same as the magnitude of the RF signal directed on to the feed forward signal path 16 by the splitter 12.
The sampled RF signal on the feed forward path 16 is applied to a phase shifter 22 that inverts or phase shifts the sampled RF signal to be 180° out of phase with the RF signal on the main signal path 14. The RF signal from the coupler 20 and the phase shifted RF signal from the phase shifter 22 are summed by a summer 24 where the main frequency in the signals is cancelled. Thus, the RF signal from the summer 24 includes only the intermodulation products of the RF signal from the coupler 20. In other words, because the phase shifted RF signal from the phase shifter 22 does not include the intermodulation products generated by the power amplifier 18, the main part of the RF signal from the coupler 20 is cancelled in the summer 24.
The intermodulation products signal from the summer 24 is amplified by an error amplifier 26. The output of the amplifier 26 is the intermodulation products of the RF signal that has a magnitude close to the magnitude of the intermodulation products in the amplified signal from the amplifier 18 that is left on the main signal path 14 after the coupler 20. The amplified intermodulation product signal from the amplifier 26 is sent through a variable attenuator 28 that reduces the intensity of the intermodulation products signal to a level more closely matched to the magnitude of the intermodulation products in the amplified signal from the amplifier 18.
The amplified and attenuated intermodulation products signal from the attenuator 28 is applied to a variable phase shifter 30 that selectively phase shifts the signal to be 180° out of phase with the RF signal on the main signal path 14. The inverted intermodulation products signal from the phase shifter 30 is then coupled to the amplified RF signal on the main signal path 14 by a coupler 32. The intermodulation products in the amplified RF signal from the amplifier 20 are thus cancelled by the phase shifted and intensity controlled intermodulation products signal from the phase shifter 30. Therefore, the output of the coupler 32 is an amplified RF signal with little or no intermodulation products. A controller 34 controls the circuit 10, and sets the attenuation provided by the attenuator 24 and the phase shift provided by the phase shifter 30.
The circuit 10 suffers from a number of problems and drawbacks. These problems include the requirement of a large number of directional couplers and signal splitters to sample the RF signal at the input, after the power amplifier 18, and at the output of the circuit 10 that increase the complexity of the circuit 10. Another disadvantage includes the requirement of the high linearity error amplifier 26 so that the feed forward path 16 does not produce significant additional spurious signals from the high power amplification process.